Leaseholds are commonly associated with flats. You will own the property, but not the land it's on. You will also have no financial stake in the building where your property is located.
Because you don't own the land, you may be asked to pay ground rent which is an extra charge you should factor in when looking at leasehold properties.
Freeholds are usually associated with houses, you'll own the property as well as the land it is on. You will be responsible for maintaining the land and property.
See more information about Freeholds and Leaseholds here.